1. Field of the Invention
The present invention relates to an IIR digital filter applied to color video signals.
2. Description of the Prior Art
An IIR digital filter is a filter having sharp characteristics at lower order. The IIR digital filter has a feedback constitution as shown in FIG. 1. As clearly seen from FIG. 1, a present input signal is supplied from an input terminal designated by numeral 1 to one input terminal of an adder 2, and a past output signal stored in a unit delay circuit 3 is multiplied by coefficient in a multiplying circuit 4 and then supplied to another input terminal of the adder 2, and the output of the adder 2 is taken through the unit delay circuit 3 to an output terminal 5.
For example, if a system function H(Z) is expressed by ##EQU1## an IIR digital filter of second order corresponding to this H(Z) is constituted as shown in FIG. 2.
In FIG. 2, input signal x.sub.1 from an input terminal 10 is supplied to an adder 11. The added output of the adder 11 is supplied to a unit delay circuit 12, and the output of the unit delay circuit 12 is supplied to another unit delay circuit 13. The output of the unit delay circuit 12 is multiplied by a multiplying circuit 14 of coefficient b.sub.1 and supplied to the adder 11. The output of the unit delay circuit 13 is multiplied by a multiplying circuit 15 of coefficient b.sub.2 and supplied to the adder 11. The output of the adder 11 is supplied to another adder 16, and the output of the unit delay circuit 12 and the output of the unit delay circuit 13 are multiplied by a multiplying circuit 17 of coefficient a.sub.1 and a multiplying circuit 18 of coefficient a.sub.2 respectively, and then both multiplying outputs are supplied to the adder 16. The output of the adder 16 is supplied to an output terminal 19, and output signal Y.sub.1 is taken from the output terminal 19.
As shown in FIG. 2, the IIR digital filter has a feedback constitution including the multiplying circuits 14 and 15. This constitution corresponds to the polynomial in the denominator of expression (1).
For example, when a color video signal is digitized by a sampling pulse of 4f.sub.sc (f.sub.sc : color subcarrier frequency), one sampling clock period is 70 n sec. If the color video signal is processed using the IIR digital filter shown in FIG. 2, the term b.sub.1 Z.sup.-1 including Z.sup.-1 in the denominator must be processed by a product/sum operation composed of multiplication and addition in a short time of 70 n sec. Since this processing requires time particularly for the multiplication, an operation element of low speed and small power consumption, such as CMOS, cannot be used, but that of high speed and large power consumption, such as a bipolar element or ECL element, can only be used.